All of CB Activity can be translated. No, it does not add strings to your language plugin; you must do this your self. The below strings are for GJ in CB Activity that you can add and translate to whatever is needed.
'created a category' => 'created a category',
'created a group' => 'created a group',
'joined a group' => 'joined a group',
'scheduled a group event' => 'scheduled a group event',
'is attending a group event' => 'is attending a group event',
'uploaded a group file' => 'uploaded a group file',
'uploaded a group photo' => 'uploaded a group photo',
'published a group video' => 'published a group video',
'posted in a group' => 'posted in a group',
'replied to a group post' => 'replied to a group post',
Basically all the "titles" seen when editing an activity in backend can be translated from cbtemplugins language file (you'll need to manually add the new strings though).